Differences from Unix

specialist hardware
A typical Plan 9 installation will have a dedicated cpu server, a dedicated file server and many dedicated terminals The file server and cpu server will be connected by the fastest link available.
"everything is a file"
Device drivers, network connections, environment variables and many other services are represented by files in the individuals file name space. This name space can be manipulated to customise the user's environment. User-level file servers are trivial to write and use, and all of this is easily distributed as the file server communication is all in a simple protocol.
minimalist philosophy
Plan 9 is an operating system for programmers. It emphasises simplicity over configurability, good design over compatibility and pragmatism over "buzzword compliance".
sensible security
There is no super-user or root. Communication with the file server is only through a simple protocol which allows no special access. Passwords are never transmitted across the network, instead the terminal manages a challenge/response session with the authentication server.
1990s user interface
A three-button mouse and bitmapped display are assumed. Support for Unicode has been included from the ground up. Character-based user-interfaces (vi, xterm, rn) have been superseded.